Literally, when somebody's brain has stopped functioning such that they will never regain conciousness, hence coloquially somebody who is idiotic, dumb or clueless beyond belief. Similar to "too stupid to live".
You dented my car, you braindead fuck!
by Linton July 11, 2003
Gory zombie horror film of extremely high calibre by Lord of the Rings' Peter Jackson and Fran Walsh. Great stuff.
Braindead is a great film, I liked the bit with the lawnmower!
by cakeandfinewine September 23, 2004
When you smoke so much pot through out your life that you become really stupid.

Synonym: Vegetable
Joe:Poor guy, he smoked way to much pot.
Bill:He's quite the vegetable.
Joe:Maybe we should wipe up the drool from his chin?
Bill:Naw,he is braindead,he will just keep drooling.
by Roflmfaoamp May 23, 2009
to fuck someone in the ears so hard that they lose braincells in the process.
Jason: Sunny, can I braindead you?
Sunny: Sure thing hun!

*30 minutes later*

Jason: You alright Sunny?
Sunny: I can bake I can bake I can bake
by the savageness April 14, 2011
Sentient Human: Prove male privilege exists.
Braindead Feminists: ARGAGHHAGHAGHRAGHRGGAGAAGAGHA
by APersonYouMightKnow April 23, 2019
What happens after shit gets 'craxy'
Shit got craxy at the party mansion. Next day Jono was wondering why he could not function when it occurded to him that he had braindeaded himself. Things are simpler for him now.
by Jono@partymansion February 18, 2008
A disease when you tend to do something stupidly and not realize what you are doing.
Guy ; I have braindeadism.
Girl ; I know, you keep hitting yourself.
Guy ; I AM!?
by CHINGPINGLING January 17, 2011